Musnad Ahmad 30:3620

'Abd al-Samad narrated to us, Hammam narrated to us, Qatadah narrated to us, from al-Nadr ibn Anas, from Bashir ibn Nahik, from Abu Hurairah, that the Prophet, peace and blessings of Allah be upon him, said: "Whoever prays one rak'ah of the morning [prayer] before the sun rises, then let him pray another one with it."